Magnetic disk drives are desired for their large capacity, and the demand for these drives is intensifying more and more for uses as so-called household information appliances such as video recorders, audio equipment, car navigation equipment, video cameras, etc., in addition to the drives' conventional application as major storage devices for computers. Accordingly, technologies for improving the areal recording density of hard disk drives have been developed. To improve the recording density, it is sometimes necessary to make the write and read track width and the read gap of a magnetic head wider. Specifically, the magneto-resistive (MR) sensor used as a read device may be made smaller including the read track width, stripe height and film thickness thereof. Here, the terms “read track width” and “stripe height” respectively represent the width and depth of the sensor as viewed from the medium facing the magnetic head.
Higher recording density may be realized by making the width and bit length of tracks recorded on the recording medium smaller. This lowers the magnitude of read signal magnetic flux generated from bits recorded on the medium. Therefore, in order to realize higher recording density, the sensitivity of the sensor used in the read head can be raised.
Until recent years, GMR (Giant Magneto-Resistive) film had been used as the sensor in a read head. The MR ratio is a parameter that can represent the sensitivity performance of a magneto-resistive sensor. The MR ratio, expressed in percentage (%), indicates the ratio of the sensor's change in resistance to the minimum resistance. In GMR sensors, the MR ratio is 15% at most.
For areal recording densities not higher than 100 Gb/in2, GMR films were used as sensors in read heads. Beyond 100 Gb/in2, however, still higher sensitivity TMR (Tunnel Magneto-Resistive) films are used as sensors in place of GMR films since GMR films are not sufficient in sensitivity.
The basic structure of the TMR sensor film is basically the same as that of the GMR film, which is commonly referred to as a spin valve, except that an insulation barrier layer is formed in place of a non-magnetic conductive spacer. The TMR sensor film has a layered structure comprising: an underlying layer; an antiferromagnetic layer; a first ferromagnetic layer which is exchange-coupled to the antiferromagnetic layer; a second ferromagnetic layer whose magnetic moment is coupled in antiparallel with the magnetic moment of the first ferromagnetic layer via an antiparallel coupling layer; an insulation barrier layer; and a third ferromagnetic layer. The first and second ferromagnetic layers are called a pinned layer. Since the magnetic moments of the first and second ferromagnetic layers constituting the pinned layer are strongly coupled in antiparallel with each other and the magnetic moment of the first ferromagnetic layer is strongly pinned due to exchange coupling with the antiferromagnetic layer, the pinned layer does not easily change the direction of its magnetic moment. The third ferromagnetic layer is called a free layer and the direction of its magnetic moment easily changes by an externally applied magnetic field.
Magnetic information recorded on a medium may be reproduced through the following process. A signal magnetic field occurring from a recorded bit enters the sensor. The signal magnetic field rotates the magnetic moment of the free layer. This changes the relative angle of the magnetic moment of the free layer with respect to that of the pinned layer. The changed relative angle changes the probability of spin-dependent electron scattering, resulting in a change in the resistance of the sensor. By converting this resistance change of the sensor to an electrical signal, the recorded bit information is reproduced.
The TMR sensor differs from the GMR sensor in that the sense current flowing through the TMR sensor is perpendicular to the sensor film while the sensor current through the GMR sensor is along the sensor film. Thus, its electrode structure for applying a current to the sensor film is also different from that of the GMR sensor. In principle, although both lie in the common phenomenon that resistance changes depending on electron-spin scattering in a magnetic structure, these electrons in the GMR sensor are conduction electrons which move in metal while in the TMR sensor they are electrons which pass through an insulation barrier layer by the tunnel effect.
FIG. 15 is an example of an estimated road map of the read track width, stripe height and areal resistivity (RA) for higher areal recording densities in the future. In FIG. 15, RA is an electrical resistance perpendicular to the sensor film surface of one μm2. Since the TMR sensor passes a current perpendicularly through the film, the resistance of the sensor is inversely proportional to the area of the sensor.
Raising the recording density results in a decrease of the sensor area through which the sense current passes since the read track width and stripe height are made smaller. Thus, if the RA remains the same, the resistance increases in reverse proportion to the sensor area. For example, if the recording density is raised from 350 Gb/in2 to 1000 Gb/in2 as is shown in FIG. 15, the resistance quadruples since the sensor area decreases to a fourth. When the resistance increases like this, the signal processing circuitry does not properly operate. Therefore, the resistance of the sensor, as viewed from the signal processing circuitry, is kept constant to avoid problems. That is, decreasing the areal resistivity RA of the sensor as the sensor is reduced in size is appropriate. In FIG. 15, RAs are calculated so as to keep constant the resistance of the sensor even when the sensor area is reduced. As shown in FIG. 15, the RA is decreased to 1.0 and 0.4 at areal recording densities of 500 Gb/in2 and 1000 Gb/in2, respectively. Although FIG. 15 is merely an estimation which may include large or small differences from true values, it represents a future trend.
As described above, for higher recording density, it is technologically logical for the sensor to have a higher MR ratio and a lower RA. In initial TMR sensors, insulation barrier layers were made of alumina or titanium oxide. Their MR ratio was about 30% while that of GMR sensors was 15% at most. This much larger MR ratio contributed to realizing higher sensitivity sensor films, resulting in commercialization of TMR heads.
Then, studies of magnesium oxide earnestly began as a material for insulation barrier layers in TMR sensors. Attention to this material is attributable to W. H. Buttler who indicated in 2001 that according to theoretical calculation, MR ratios beyond 1000% may be realized by a structure comprising a (001)-oriented MgO layer sandwiched between (001)-oriented iron layers. Further, Yuasa et al. disclosed that MgO (001) is grown conformably on single-crystal Fe (001) by using the MBE method, exhibited 180% at room temperature, the highest MR ratio at that time.
Although such a high MR ratio was attractive in view of many desired applications, the proposed structure comprising MgO grown conformably on single-crystal Fe was difficult to directly apply to electronic devices, such as magnetic heads and MRAMs, since the sensor must be formed on a polycrystalline magnetic shield.
According to Japanese Patent Office (JPO) Pub. No. JP-A-2008-135432, high crystallinity MgO can be deposited on an amorphous CoFeB alloy film by a sputtering method to obtain, by annealing it, a high MR ratio of 180% at room temperature. However, its areal resistivity is as high as 1000 Ωμm2, according to this reference, leaving a problem to be solved before the method can be applied to hard disk read heads.
Therefore, studies have been made to attain a MgO-used TMR sensor having a lower RA and improved MR ratio, aimed at applications like hard disk read heads. In K. Tsunekawa et al., “CoFeB/MgO/CoFeB Magnetic Tunnel Junctions with High TMR Low Junction Resistance”, InterMag 2005, FB-05, Apr. 7, 2005, a method for fabricating an insulation barrier layer is disclosed. In this method, after a thin metallic magnesium layer of approximately 0.4 nm is deposited, a magnesium oxide layer is formed by a RF sputtering method with a MgO target. This method has realized a high MR ratio of 100% with an RA of 2.0 Ωμm2.
In this document, the interlayer-coupling magnetic field Hint is also considered. The interlayer-coupling magnetic field Hint is a magnetic field which the free layer receives from the pinned layer. Since the free layer is always subject to the interlayer-coupling magnetic field Hint, this magnetic field, if large, affects the free rotation of the free layer's magnetization and consequently lowers the symmetry property and intensity of the read signal waveform. In this document, the Hint increases as the RA decreases (that is, the thickness of the MgO layer decreases) and reaches to as large is 80 Oe at an RA of 2.0 and greatly exceeds 100 Oe at an RA of 1.0. The inventors predicted that if the RA is further decreased below 1.0, the Hint would steeply increase and reach to 200 Oe and it would become necessary to decrease the Hint in such a low RA region. In this connection, in the case of GMR head sensors, the interlayer-coupling magnetic field was controlled to 30 Oe at its highest.
In K. Tsunekawa et al., “CoFeB/MgO/CoFeB Magnetic Tunnel Junctions with High TMR and Low Junction Resistance”, InterMag 2005, FB-05, Apr. 7, 2005, an antiferromagnetic PtMn layer is deposited on an underlying Ta layer. However, this structure is almost never used in TMR heads due to poor practicality since the magnetization of the pinned layer is not sufficiently pinned. In the current structure employed in commercialized versions, the underlying layer is a layered film composed of Ta and Ru layers, Ta and NiFeCr alloy layers, or Ta and NiFe alloy layers, the anti ferromagnetic layer is made of MnIr alloy, the first ferromagnetic layer is made of CoFe alloy, and the antiparallel coupling layer is made of Ru.
In the currently used above structure whose underlying layer is a laminated film composed of Ta and Ru layers, Ta and NiFeCr alloy layers, or Ta and NiFe alloy layers, the antiferromagnetic MnIr alloy layer has a face centered cubic (fcc) crystal structure with the orientation (111) grown preferentially parallel to the film surface. In JPO Pub. No. JP-A-2008-60273 and Jap. Pat. No. 3083237, it is described that the orientation (111) of the fcc structure of MnIr is used and preferable, respectively.
Usually, out of all of the crystallographic planes, the closest-packed plane has the smallest interfacial energy. Therefore, crystal generally grows so that the closest-packed plane forms the surface. FIGS. 16A-16B show crystallographic conformability relations. Since Ru has a hexagonal close-packed (hcp) crystal structure and its (001) plane is closest-packed, Ru is preferentially (001)-oriented. MnIr on Ru is (111)-oriented since MnIr has a face-centered cubic crystal structure, its (111) plane is closest-packed and, as shown in FIG. 16A the lattice mismatching of the (111) plane with Ru (001) is as small as 1.1%. In the case of NiFeCr or NiFe, its crystal system is fcc and the (111) plane is closest-packed. Therefore, NiFeCr or NiFe is preferentially (111)-oriented and MnIr thereon is also (111)-oriented since MnIr also has a fcc crystal structure, its (111) plane is closest-packed, the lattice mismatching of the (111) plane with NiFeCr or NiFe is about 5.9%, relatively small, and the interfacial energy of the MnIr (111) plane is small.
As mentioned earlier, to realize higher recording density in the future, the RA can be lowered to 1.0 Ωμm2 or lower. Lowering the RA below 1.0 Ωμm2 remarkably enlarges the Hint. Therefore, suppressing the Hint is one way to help achieve a lowered RA.
As a generation mechanism of the Hint, it is put forward that the Hint is attributable to the waviness of the interfaces of the insulation layer with the ferromagnetic layers. Its theory is illustrated in FIG. 17. If the waviness is large, magnetic poles are generated at the interfaces. A Hint is generated as a result of interaction between magnetic poles (190 and 200 in FIG. 17) at the respective interfaces of the pinned layer 160 and free layer 150. This theory is known as Neel's orange peel coupling. If the MgO layer is thinned to decrease the RA for meeting the demand for higher densities, this sharply enhances the interaction between magnetic poles, the interaction being generated due to the waviness of the interfaces with the pinned layer and free layer, and thus sharply enlarges the Hint. For example, J. C. S. Kools et al., “Effects of finite magnetic film thickness on Neel coupling in spin valves”, J. Appl. Phys., Vol. 85, No 8 (1999), p. 4466-4468, describes this interaction in spin valves.
In addition, as shown in K. Tsunekawa et al., CoFeB/MgO/CoFeB Magnetic Tunnel Junctions with High TMR and Low Junction Resistance”, InterMag 2005, FB-05, Apr. 7, 2005, if the MgO layer is thinned enough to decrease the RA to below 2.0 Ωμm2, the MR ratio sharply decreases. Below 1.0 Ωμm2, the sharp decrease of the MR ratio is very remarkable. Suppressing this decrease of the MR ratio is another technique that can help realize higher recording density.
Therefore, a technique which can suppress the Hint and suppress the decrease in MR ratio would be very beneficial to the manufacturing of read head sensors with higher areal densities.